1. Field of the Invention
The present invention relates to an alloy sheet for making a shadow mask having high press-formability and method for manufacturing thereof.
2. Description of the Related Art
Recent up-grading trend of color television toward high definition TV has employed Fe--Ni alloy containing 34 to 38 wt. % Ni as the alloy for making a shadow mask to suppress color-phase shift. Compared with low carbon steel which has long been used as a shadow mask material, conventional Fe--Ni alloy has considerably lower thermal expansion coefficient. Accordingly, a shadow mask made of conventional Fe--Ni alloy raises no problem of color-phase shift coming from the thermal expansion of shadow mask even when an electron beam heats the shadow mask.
Common practice of making the alloy sheet for shadow mask includes the following steps. An alloy ingot is prepared by continuous casting process or ingot-making process. The alloy ingot is subjected to slabbing, hot-rolling, cold-rolling, and annealing to form a alloy sheet.
The alloy sheet for the shadow mask is then processed usually in the following steps to form shadow mask. (1) The alloy sheet is photo-etched to form passage-holes for the electron beam on the alloy sheet for shadow mask. The thin alloy sheet for shadow mask perforated by etching is hereinafter referred to as "flat mask". (2) The flat mask is subjected to annealing. (3) The annealed flat mask is pressed into a curved shape of cathode ray tube. (4) The press-formed flat mask is assembled to a shadow mask which is then subjected to blackening treatment.
The shadow mask which is prepared by cold-rolling, recrystallization annealing, or by further slight finishing rolling after recrystallization annealing, has higher strength than conventional low carbon steel. Accordingly, such a conventional Fe--Ni alloy is subjected to softening-annealing (annealing before press-forming) at a temperature of 800.degree. C. or more before press-forming to make grains coarse. After the softening-annealing, an warm-press is applied to carry spheroidal forming. The temperature of 800.degree. C. or more is, however, in a high temperature region. Therefore, from the view point of work efficiency and economy, the development of manufacturing method to obtain such a low strength as in the material, which is softening-annealed at 800.degree. C. or more, by the softening-annealing at 800.degree. C. or less has been waited. Responding to the request, a prior art was proposed in JP-A-H3-267320 (the term JP-A- referred to herein signifies unexamined Japanese patent publication ). The prior art employs cold-rolling, recrystallization annealing, finish cold-rolling and softening annealing. The finish cold-rolling is conducted at a reduction ratio of 5 to 20%. The temperature of the softening annealing is below 800.degree. C., more specifically at 730.degree. C. for 60 min. The prior art produces a sheet having sufficiently low strength to give good press-forming performance with the 0.2% proof stress of 9.5 kgf/mm.sup.2 (10 kgf/mm.sup.2 or less) at 200.degree. C.
However, the prior art does not satisfy the quality required to perform a favorable warm press-forming. Shadow masks prepared by the prior art were found to gall the die and to,generate cracks at the edge of shadow masks.
Nevertheless, cathode ray tube manufacturers try to carry the softening annealing at a lower temperature and in a shorter time than conventional level described above aiming to improve work efficiency and economy. The target annealing time is 40 min. or less, and in some cases, as short as 2 min. However, if such an annealing condition is applied to the prior art, the galling of dies during press-forming becomes severe and the crack on shadow mask increases to raise serious quality problem.